The invention is related to the field of radiography using protons or light ions (generally hadrons), referred to herein as “proton radiography”.
A principal application of medical proton radiography is for deriving patient-specific proton stopping power images with which to guide proton therapy treatment planning. The principal advantage of proton and light ion therapy (collectively, “hadrotherapy”) relative to conventional X-ray photon-based radiation therapy is the ability to more precisely shape the dose delivery profile so as to intensely irradiate target tissue while sparing non-target organs at risk. This is typically performed using the sharply falling distal edge of the Bragg peak, where hadrons deposit rapidly increasing amounts of energy in target tissue before suddenly coming to a stop, thereby sparing tissues beyond the stopping point.
In present practice, the ideal capability of proton therapy is undercut by insufficient knowledge of precisely where this stopping point occurs along a given line-of-incidence within a given patient for an incident proton beam with a defined initial energy. Current treatments typically employ treatment-planning X-ray CT scans, but this is not ideal. X-ray stopping power is generally correlated with but not precisely equivalent to proton stopping power, and a particular patient's anatomy may have changed between treatment planning scans and treatment delivery sessions. Information confirming expected proton stopping power along a line of response for a given patient could instead be obtained by increasing proton incident energy so as to generate a proton beam capable of transmission through the patient and out the patient's opposite side, with the residual proton energy measured after exiting the patient. Subtracting this transmitted proton remaining (“residual”) energy from the incident energy then gives information on the proton stopping power along the line of incidence. This is proton transmission radiography, and by combining a complete set of lines of response across the patient one could perform proton computed tomography (pCT) to form a 3-dimensional slice image of the proton stopping power throughout an entire slice through the patient.
Proton radiography and proton CT, despite their acknowledged potential utility for proton radiation therapy planning and potential dose delivery modifications during a proton therapy delivery session, are not presently in widespread clinical use. This is largely because prototype proton radiography designs to date are bulky, rate- and flux-limited, slow, expensive, and difficult to incorporate into the clinical environment.
A review article by Poludniowski G, Allinson N M, and Evans P M entitled “Proton radiography and tomography with application to proton therapy.” Br J Radiol 2015; 88. 20150134, describes that a proton transmission radiograph can be obtained by directing a proton beam through an object and onto a suitable sensor. The passage of protons is detected indirectly, typically exploiting its transfer of energy via ionization and excitation. The definition of proton-integrating technology is that signal (e.g. in a pixel) is due to the passage of an undetermined number of incident protons. The resulting signal will depend on both proton fluence and energy distribution, but proton integrating radiography assumes that the signal can be calibrated to average proton water equivalent path length (WEPL) through the patient. The limitations of the proton-integrating approach arise from the interplay of multiple Compton scattering (MCS) and energy loss effects, resulting in a “halo” effect at material interfaces. The degradation in spatial resolution for integrating compared with tracking systems will depend on the patient anatomy and the detector-patient geometry.
The above-referenced review describes both proton-integrating radiography systems and proton-tracking systems. By contrast to proton-integrating devices, proton-tracking radiography and tomography systems consist of a number of position-sensitive detector (PSD) modules to infer proton path (typically between one and four), as well as a residual energy range detector (RERD) to determine proton residual energy. The precision of WEPL determination can be improved by increasing the number of protons in an acquisition. The standard error on an estimate of WEPL will decline by the square root of the number of protons measured. Increasing proton number does, however, increase patient imaging dose and scan acquisition time
The above review article lists ten current and recent proton radiography (pRG)/proton CT (pCT) prototypes. In particular, the review article identifies several types of residual energy-range detector technology as follows: Plastic scintillator telescopes (including “hybrid” devices), NaI(Tl) or CSi(Tl) or YAG:Ce calorimeters, x-y Sci-Fi [Scintillating Fiber] arrays, and CMOS APS [Active Pixel Sensor] telescopes. A calorimeter is described as determining the energy of the outgoing proton and therefore accurately determining its state immediately after traversing the patient. In a range telescope, however, only the stopping depth of the proton is determined. Since there will be statistical variations in penetration depth within the range telescope itself (residual range straggling) this will contribute extra uncertainty on the estimate of WEPL [Water Equivalent Path Length]. While this is true, a calorimeter will in fact always possess a finite energy resolution. In addition, calorimeters have the fundamental limitation that when applied to newer proton-beam delivery systems (e.g. synchrocyclotrons and other systems with small delivery “duty cycles”) that deliver temporally narrow “bunches” of protons rather than individual protons separated in time, the calorimeter response varies in proportion to the uncertain and variable number of protons in each individual “bunch”, In consequence, the superiority of any particular RERD [Residual Energy-Range Detector] over another cannot be established based on such a general criterion.
In addition to limited accuracy, the calorimeter, range telescope, and hybrid technologies described in the review article suffer from the following additional deficiencies: cost, complexity, sensitivity to radiation damage, and bulky volumes incompatible with treatment delivery and patient positioning geometries. An additional deficiency of the above technologies is their limited speed both for detection and for readout, which drives up cost and complexity by requiring fine segmentation to avoid requiring low proton beam fluxes and consequent overlong radiographic scan times.